Surface Tension and Vapor Pressure

surface tension picture

Core Concepts

In this article, you will be introduced to surface tension and find out what is vapor pressure. In addition, you will learn interesting applications of each to see how chemistry is everywhere in the real world!

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What is surface tension?

Surface tension is a measure of the intermolecular forces present at the surface of a liquid. An easy way to think about it is the amount of force that is required to break the surface of a liquid.

A liquid, such as water, with hydrogen bonding present will have high surface tension. Because hydrogen bonds are so strong, there will be a higher amount of force needed to break the bonds and distort the surface barrier of the liquid. On the other hand, liquids with weaker intermolecular forces, such as benzene, correspondingly have low surface tension. These types of liquids contain bonds that can be broken more easily, so the surface is more likely to be distorted.

surface tension example

What is vapor pressure?

Vapor pressure is the pressure exerted by the vapor of a substance when it is in equilibrium with its solid or liquid phase. This property is temperature-dependent; the temperature at which this pressure is equal to atmospheric pressure is the boiling point of the substance in question. In addition, evaporation rate also corresponds, higher vapor pressure means a higher evaporation rate.

Vapor pressure is also dependent on the amount of each substance present based on Raoult’s Law pictured below:

Vapor Pressure in Mixture = Mole Fraction * Pure Vapor Pressure

Interesting applications of Surface Tension & Vapor Pressure

Surface tension is what lets bugs skate across the surface of the water; since their weight is less than the surface tension, they can glide without breaking the hydrogen bonds enough to sink through. Droplets are also a result of this property, droplets together counter to gravity!

The separation of different liquids is also achieved using vapor pressure. Some liquids will evaporate faster than others and as a result, the vapor will condense in a different concentration as the starting liquid. This is known as distillation, which can come in the form of simple or fractional distillation, and is a common purification technique used in organic chemistry labs.

vapor pressure example

Further Reading